224 LECTURES ON MOLLUSCA. 



Tribe I. Geophila. Land Sn ails. 



Family Helicidje. (True Snails.) 



The true snails have their body distinct from the foot, and protected 

 by a spiral shell. The shape of this is extremely variable, presenting 

 differences much greater than is usual between widely distinct families 

 in the marine tribes. Yet the different forms pass into each other by 

 such insensible gradations, and the animals are so like in all essential 

 particulars, that the division into genera is a matter of great difficulty. 

 There are many myriads of species from all parts of the globe, and 

 from all kinds of habitats. Many species have been found on moun- 

 tains from 8,000 to 11,000 feet high, both in the Old and New World, 

 while others live in marshes, or on the sea-shore. In some few groups, 

 both animal and shell present well-marked peculiarities ; others are 

 restricted to special districts ; but in general the sections are constituted 

 for the convenience of identifying species. How long the snails have 

 lived on the surface of our globe it is impossible to say, as the remains 

 entombed in rocks are almost exclusively of aquatic productions. 

 Nevertheless many snails have been washed down into tertiary strata; 

 and it is singular to find forms and even species now peculiar to the 

 New World, such as Megaspira, Proserpina, Glandina, and Slenotrema, 

 fossil in the European Eocene ; showing that existing forms have long 

 outlived existing continents. The oldest snail known is a little Pupa, 

 found by Prof. Dawson, in situ, on the fossil trees in the coal measures 

 of Nova Scotia ; generically it exactly resembles existing forms. 



The " horns" of the snails are in reality very long and sensitive eye 

 stumps. The true tentacles are short, and nearer the mouth. They 

 have a saw-like upper jaw to bite the leaves, and plain teeth arranged 

 in squares. The nose, or lung valve, is just under the right side of 

 the shell; the reproductive orifice under the right eye stalk. Some of 

 the European species form and dart out minute needles, it is supposed 

 to attract their mates. The old genera of Lamarck may be taken as 

 sections, from which the immense multitude of species now known re- 

 quire to be subdivided. 



The true snails have a short spire, and a mouth rather broader than 

 long. The eatable snail, Helix pomatia, (which is believed to have 

 been introduced into South Britain by the Romans for epicurean pur- 

 poses,) and its congeners, have a semicircular mouth and rather thin 

 lip. Eurycratera has a thin shell and very capacious body whirl. 

 Hclicostyla comprises the tall, compact snails of the Philippines. 

 Acavus, which abounds in the Old World, has the mouth somewhat 

 produced in front, and the lip thickened all round, without umbilicus. 

 The group Caracolla has the lip continued all round, the spire flattened 

 and generally keeled. In Lucerna the mouth is more or less twisted, 

 with teeth; and in Anostoma the adult shell is turned upside down, 

 the mouth joining the apex. Lychnus is an Eocene Anostoma without 

 teeth. Tridopsis contains the ordinary American toothed snails; the 

 flat, many-whirled forms being called Polygyra. Geotroclius contains 

 the conical, thin, flat-based snails, shaped like Calliostoma. Solariop- 

 sis contains the snake-skin snails of tropical America. Macrocyclis 



